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415: The Most Dangerous Number

There’s only one metric that really matters — and it’s telling us we’ve done less than zero to combat climate change

The coal-fired Plant Scherer, one of the nation's top carbon dioxide emitters, stands in the distance in Juliette, Ga., Saturday, June, 3, 2017. U.S. President Donald Trump declared Thursday he was pulling the U.S. from the landmark Paris climate agreement, striking a major blow to worldwide efforts to combat global warming and distancing the country from its closest allies abroadTrump Climate Georgia, Juliette, USA - 03 Jun 2017

The coal-fired Plant Scherer, one of the nation's top carbon dioxide emitters, in Juliette, Georgia.


Last week, an exquisitely sensitive instrument located in a metal shack on the top of Mauna Loa, a 13,679-foot-high volcano in Hawaii, recorded a terrifying human achievement: Thanks to our ever-increasing addiction to burning fossil fuels, the level of carbon dioxide in the Earth’s atmosphere has risen to 415 parts per million. This is the highest level it has been since human beings have lived on Earth. And it is further evidence (as if further evidence were needed) of just how hell-bent we are on cooking the planet we live on.

For the planet itself, 415 ppm is no BFD. Over the past 4 billion years or so, it’s been much, much higher. But for us humans, 415 is a very dangerous number. The last time CO2 levels were at 415 ppm, during the Pliocene period about 3 million years ago, there was plenty of life on Earth, but the Earth itself was a radically different place. Beech trees grew near the South Pole. There was no Greenland ice sheet, and probably not a West Antarctic ice sheet, either. Sea levels were 50 or 60 feet (or more) higher.

That’s the world we’re creating for ourselves by pushing carbon dioxide levels to 415 ppm. Right now, a lot of atmospheric warming is being absorbed in the oceans. But those oceans are like a big flywheel, and the heat will be radiated out. That means, among other things, goodbye ice sheets, hello condo diving in Miami.

One way to think about carbon dioxide levels in the atmosphere is as a thermostat for the planet. As you’ll remember from third-grade science class, carbon dioxide is exhaled by animals, including humans, and inhaled by plants. It is also released when plants and animals decay, volcanoes erupt, and, most importantly, when we burn fossil fuels. Last year, we dumped about 37 billion tons of carbon dioxide into the atmosphere by burning fossil fuels. The more coal, oil and gas we burn, the faster that number rises. Before the Industrial Revolution, the level of carbon dioxide in the atmosphere was 280 ppm. Sixty years ago, it was 315 ppm. For the past few years, it has been rising by about 2 or 3 ppm a year.

That might not sound like much. However, carbon dioxide molecules happen to be very good at trapping heat in the atmosphere. Scientists have understood this very well since the 19th century. Carbon dioxide molecules are like the prison guards of the Earth’s atmosphere — they let sunlight in, but they don’t let heat out. Scientists argue about exactly how efficient carbon dioxide is at warming the Earth, but there is basic agreement that a doubling of carbon dioxide levels from 280 ppm will warm the Earth’s atmosphere by 2 to 3 degrees Celsius.

Right now, at 415 ppm, the climate has already warmed about one degree. And you don’t need to be a scientist to see the impacts: glaciers are melting, sea levels are rising, wildfires are growing more frequent, hurricanes and typhoons are growing more intense. The changes are so fast and so profound that, as a recent U.N. report suggested, more than a million species of animals and plants are at risk for extinction.

Measuring the level of carbon dioxide in the atmosphere is one of the great and underappreciated scientific accomplishments of our time. And most of the credit goes to Charles Keeling, a quiet geochemist from Scranton, Pennsylvania, who, in the 1950s, wondered if it would be useful to have a baseline measurement of the levels of carbon dioxide in the Earth’s atmosphere. In the 1950s, the heat-trapping properties of CO2 were well understood, but the link between burning fossil fuels and climate change was still tenuous. And actually measuring carbon dioxide levels in the atmosphere was more complex than it seemed, largely because carbon dioxide is a trace gas, measured in parts per million, and because the levels of carbon dioxide vary with local conditions, depending on plant life and the emissions of nearby power plants and other fossil fuel burning devices.

In 1955, Keeling camped out at Big Sur State Park in California, collecting samples of air in flasks to measure their carbon dioxide content. Three years later, seeking even clearer air, he lugged his instruments up Mauna Loa in Hawaii and began taking measurements there.

Keeling quickly made two important discoveries. The first was that the Earth, in effect, breathes. As the seasons change, carbon dioxide levels in the atmosphere oscillate slightly. He immediately understood that this was the result of the seasonal growth of plants, especially trees. As trees sprouted leaves and grew over the spring and summer, carbon dioxide levels fell slightly. As the trees shed leaves in the fall and winter and the leaves began to decay, the carbon dioxide levels rose.

His second, and more ominous and consequential finding was that each year, the peak level of carbon dioxide in the atmosphere was a little higher than the year before. And that increase, it was clear, was due to the combustion of fossil fuels.

“It became clear very quickly that his measured CO2 increase was proportional to fossil fuel emissions and that humans were the source of the change,” James E. Hansen, director of NASA’s Goddard Institute for Space Studies, told the New York Times. “He altered our perspectives about the degree to which the Earth can absorb the human assault.”

Sixty-one years have now passed since Keeling took his first measurement on Mauna Loa (Keeling died in 2005; the work has now been taken over by his son, Ralph Keeling, a professor at Scripps Institution of Oceanography, which operates the Mauna Loa observatory), and the data over those years can be charted in what’s now known as the Keeling Curve, which measures the yearly growth of carbon dioxide levels in the atmosphere.

When you look at this curve, two things are obvious. First, it is a smooth upward curve, with no breaks or dips or plateaus. Despite the decline in the cost of solar power, despite all the climate marches in the streets, despite the wildfires and melting glaciers and increasing summer heat, it is very obvious that, by the only metric that really matters, we have done less than zero to reduce the levels of carbon dioxide in the atmosphere.

The second thing you notice is that not only is the curve continuing to rise, but it is rising faster than ever before.

Among other things, the Keeling curve is a perfect record of Homosapiens’ self-destructive impulse. We have known for precisely 61 years now that burning fossil fuels is warming the Earth’s atmosphere and putting the stability of our Goldilocks climate – the not-too-warm-not-too-cold world that has allowed humans to thrive over the last 10,000 years – at risk. And we have done nothing about it. In this sense, the Keeling Curve may turn out to be a kind a civilizational snuff film, a gruesome story, told in cool scientific fact, of our destruction of life as we know it on Earth.




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